Mark Houlahan
Brand Manager, Mustang Monthly
December 21, 2010

If you think back, probably one of the first things you did when you got your license was to find the nearest parking lot or group of friends, and lay some rubber to show off. While few of us have "grown up" since then, a nice peel out/burnout/whatever-you-call-it always puts a smile on people's faces-except for Johnny Law, of course. Most likely, though, after that first time you may have noticed only one tire mark on the ground. Call it the one-wheel-peel or one-tire-fire, either way you only had one tire clamoring for traction back there. This is probably due to the standard, or "open" rear that many cars came with originally, and was most likely the cause of some ribbing from friends and fellow gearheads. You needed to fix this issue, and fix it fast. Usually, the fix entailed swapping out the open rear for Ford's venerable Traction-Lok limited-slip unit, or if you were packing a 9-inch, you could upgrade to the Detroit Locker for some real fun, but that was about it back then.

Today is a different story. The aftermarket choices for Ford 8-inch and 9-inch differentials are much broader, bringing to the market such technologies as electric and pneumatic locking differentials, exotic friction materials, and more. Sure, the Traction-Lok is still around, and a good value for the right builds. The Detroit Locker is still hanging tough too, and available in several designs now. But they've been joined by some pretty big names in the aftermarket driveline parts arena. You've most likely heard of names like Auburn, Powertrax, Eaton, and others.

We dug through all of the choices to show you just what is available for the Ford 8-inch and 9-inch removable carrier rearends. We also delved into the late-model 8.8-inch rearend a bit too, as it's becoming a popular budget rearend swap for classic Fords and Mustangs alike. But first, we're going to walk you through the various differential types and their primary functions, so you'll be better prepared to make a purchase decision based upon your classic Ford's needs, be it street car, drag car, or road racer.

Why You Need A Differential


As your Ford travels down the road both rear wheels rotate at the same speed because they are traveling the same distance. When making a turn, the inside wheel, that is the wheel closest to the inside of the turn, is going to spin slower than the outside wheel due to the smaller circle, and thus distance, that the tire is following. Actually, all four of your tires take different paths when navigating a turn if you want to know the truth, but we're only concerned about the rear axle here, as the fronts are independent of each other in this regard.

Anyway, as the rear tires have different wheel speeds during the turn, there must be a way to separate the left and right wheels from each other, otherwise driveline strain and tire wear/noise would increase to unacceptable levels (though we know of plenty of people that love the sound of a tire chirping in a tight turn with a locker or spool, but we digress). To do this, the rear axle is designed with a differential; a device that can differentiate the incoming power from the driveshaft between the two different wheel speeds when needed.

Types Of Differentials


First off, we have the open differential, as discussed in our opening paragraph. This is essentially an iron or steel case with two (sometimes four) pinion gears, two side gears, and of course, the ring and pinion gearset. This sounds great in theory, and probably works quite well on a car with low performance on a perfect section of road. But start putting some power to the rear wheels or drive through less than perfect traction situations like wet roads, snow, or mud, and you get the "one-wheel-peel." Why? It's quite simple really. When you have an open differential and one wheel has the smallest amount of traction loss (slippage), the open differential sends all of the incoming power to the slipping wheel because it has no way of enacting force on the non-slipping wheel. This prevents the car from ever getting or regaining traction. Adding a limited slip, locker, or spool, will solve this problem and each one has its specific way of solving said problem.

A limited-slip style of differential is the most commonly known upgrade that combats the effects of tire spin with an open differential. Ford has called its version the Equa-Lok, and later the Traction-Lok. Both use a combination of alternating steel and friction clutch plates, like in an automatic transmission, under spring tension. The spring helps "lock" the axles together for traction to both wheels by exerting pressure on the clutch discs, but the alternating clutch plates allow sufficient slippage during cornering without noise. This is the system most of us are familiar with, but there are other limited-slip offerings in the aftermarket, many of which use a similar series of clutches and/or springs to provide optimum power to both wheels, while allowing slippage for cornering. The true downfall of a limited slip is the wear induced by the clutches (though a limited slip is usually able to be rebuilt with a "clutch kit") and the fact that, while the traction is split to both rear wheels, slippage can still occur, especially as the clutches wear or the spring(s) loses tension. You can actually overheat a limited slip and burn the clutches and ruin the spring(s). Clutch-based limited slips also require a friction additive to the gear oil, though many gear oils today have the additive properties built in. It's something to remember if your Ford has a limited-slip differential installed.

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Next up we have the lockers, with the Detroit Locker being one of the more common one Fordophiles may have heard bandied about in car magazines and web forums. A locker, for lack of a better description, locks the two rear wheels together, but still allows for differentiation in turns. The differentiation occurs when the inside wheel and outside wheel speeds are different and the axles overcome a preload spring (or springs) and allow the locker to unlock. A locker does direct engine power to the wheel with the most traction, unlike an open or limited-slip style of differential, but lockers are notoriously noisy and that can turn some people away. They have a tendency to ratchet or "click" while turning and sometimes their disengagement to allow differentiation at low speeds sounds like you're breaking parts back there!

Finally we come to the spool. A spool is nothing more than a solid piece of steel (for the most part) that the ring gear is bolted to and the axles are engaged to for 100 percent of the engine's power to both rear wheels. There's no differentiation whatsoever for street use. This lack of differentiation can cause driveline stress/breakage, and extreme tire wear when used on the street. A spool is most at home on the dragstrip where you want all your traction in a straight line and a little tire chirp when turning onto the return road is no big deal. I'm sure you've had a pal or heard cruise night stories of some Mustang with insane power and running a spool on the street, but other than for bragging rights, it's not the easiest thing to have under your Mustang for street use.

Limited Slip

Auburn Gear offers several differentials for Ford axles. The Auburn is a high-bias, limited-slip differential that uses a cone-shaped friction surface clutch arrangement that is coupled to the side gears, and uses centrally mounted preload springs. The preload springs, as well as the normal gear separation forces of the incoming torque, seat these two cones into the differential case to maximize torque output to both wheels. During cornering, the torque input decreases, allowing the cone clutch to slip and differentiate the wheel speeds. Auburn's limited-slip lineup includes the original Auburn High Performance limited slip, the Auburn Pro Series (with higher bias), and the Auburn ECTED Max electronic differential (pronounced Ek-Ted). The original Auburn and the ECTED Max are currently only available for the late-model 8.8-inch axle, in both 28- and 31-spline configurations. The Auburn Pro, in both 28- and 31-spline part numbers, is available for the 8-inch and 9-inch rear, as well as the 8.8-inch late-model (including a 33-spline option for optimum strength on the street).

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If you're working on an 8-inch rear, a popular upgrade is the Currie TSD (Torque Sensing Differential) for 28-spline applications. The Currie folks designed their own limited slip by machining a Traction-Lok housing to accept four spider gears instead of the stock two gears, and then they seal it up with a Currie-designed, nodular iron Traction-Lok cover. Being based on the OE Traction-Lok means that the TSD uses friction clutch plates and a bias spring to control power output and allow differentiation while turning. The Currie TSD is also available for the 9-inch axle in both 28- and 31-spline as well.

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The Detroit TruTrac, today an Eaton Corporation brand, is a different kind of limited slip in that it uses a helical-cut gearset to allow quiet open differential movement while turning, yet when traction is needed, it will slow the spinning wheel and distribute power to the wheel with the best traction. This is done without the use of wear items like clutches and springs, too. The side gears have pinion gears that ride in pockets of the differential housing, and when traction is lost, these pinion gears move outward, wedging themselves into these pockets to slow the spinning wheel. As the input torque is increased, so is the separating force that wedges the pinion gears. The TruTrac is available for the 8-inch, 9-inch, and the 8.8-inch axle.

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The Eaton Posi is only available for the 8.8-inch in Ford circles, so we'll discuss it only briefly here. Essentially, the Eaton Posi is a friction-plate-clutch style of limited slip, upgraded with carbon-fiber clutch discs, and preloaded by a central spring assembly. Like other clutch-based limited slips, when torque input increases, the clamp load on the clutches increases, delivering the power to both rear wheels before there's any chance of tire spin. Also, like many limited slip units, the Eaton Posi is rebuildable.

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Another pair of helical-gear, limited slips offerings are the Torsen T-1 and T-2 differentials. The T-1 fits the 9-inch Ford and the T-2 (and T-2R race model) fit the 8.8-inch Ford. The Torsen design differs between the two versions. The T-1 is a crossed-axis helical gear, meaning the pinion gears cross the differential's side gears instead of running parallel. The T-2 uses a parallel gear arrangement, like other helical gear differentials, which offers quieter operation, finer gear mesh, and low backlash. Both differential styles are a full-time torque sensing/biasing design, and torque is biased instantly between the two rear wheels without the use of clutches, preload springs, or special gear oil additives.

As noted earlier, the majority of our readers will be most familiar with Ford's Traction-Lok limited slip, which has been a factory option in the 8-inch, 9-inch, and 8.8-inch axles for decades. The factory Traction-Lok, like most factory parts, is designed for quiet operation and long life, something that the OEMs do for customer satisfaction. Yet, these very reasons also mean the Ford Traction-Lok is not one of the stronger limited slips either. The Ford Traction-Lok uses a series of clutch plates and a central bias spring like many limited slips. Over the years, rebuilders have used carbon-fiber clutch discs, machined the housing for additional discs, tried different preload springs, and more. They will work up to a point, but if you're pushing a lot of power there are better options out there for your Ford axle.

Yukon Gear & Axle has two different limited slips (and a locker we'll discuss shortly) in its huge catalog of driveline parts that include axles, gears, differentials, and more. The Yukon Trac-Loc is available for the 8-inch and 9-inch axles and is similar in design to the OEM Traction-Lok assembly, but stronger in every way. Yukon starts out with a nodular iron differential case, then stuffs it full of forged internal gears, high-friction composite clutches, and a high-bias spring assembly. The differential is then sealed via a billet steel hat. The Trac-Loc is available in several part numbers for 28- and 31-spline axles, and also in an HD version, which has a higher torque bias giving way to a little clutch chatter when cornering. The Yukon Dura Grip (shown here) is only available for the 8.8-inch when it comes to Ford applications, and like the Trac-Loc, is built around a nodular case with 4320 forged steel internals, composite clutches, and four high-bias springs for both 28- and 31-spline late-model axles.

Lockers

We could argue that the ARB Air Locker is more of a spool than a locker, since it doesn't allow differentiation when engaged, but since it is called a locker by name and can be turned off, we placed it under the locker subheading. The ARB is made in Australia and uses compressed air to engage an internal tooth piston within the differential to lock the side gear to the case, which locks the axles together. Of course, this requires a small on-board air compressor or other air source for it to work. Originally designed for off-road use, the ARB Air Locker has gained traction (sorry) in the performance automotive aftermarket as an upgrade for serious street and drag race use. There is no differentiation, as the ARB acts like a spool when engaged. The differential housing is made of nodular iron and the internals are nickel alloy steel with aerospace coatings on the gears. With fewer moving parts, the ARB has a reputation on and off the road for strength. Unfortunately, it is only available for the 8.8-inch in 28- and 31-spline configurations, and for the 9-inch in 31- and 35-spline

We're putting the Auburn ECTED Max under lockers as well because it's one of the only differentials that we know of that can also be electronically controlled from a limited-slip state (off) to a full locker (on). It does this by way of an electro-magnet that creates a lateral movement within the gearcase, compressing the clutch pack and locking the side gears to the differential. The ECTED Max is manufactured from aircraft-quality 9310 heat-treated billet steel for optimum strength. Because it uses an electrical connection, the locker simply needs an electrical source routed to the axle housing. No air system or mechanical linkage is needed to engage it, plus it's quiet, with no ratcheting sound like typical lockers have. The ECTED Max is currently available for the 8.8-inch late-model axle only in both 28- and 31-spline configurations.

Our last locking differential we'll look at is the Yukon Gear Grizzly Locker. The Grizzly is a tough mechanical locking differential made with a forged steel differential case and 8620 triple alloy steel internals. Similar to other mechanical locking differentials, the Grizzly Locker uses a central pinion locked to the differential case that the axle side clutches engage with, giving optimum straight line traction, yet allowing differentiation in turning. The Grizzly Locker is available for both the 8-inch and 9-inch axles in 28- and 31-spline options, as well as a 35-spline version just for the 9-inch.

The grand daddy of lockers has got to be the Detroit Locker. Now owned by Eaton Corporation, the Detroit Locker (also called the Detroit No Spin in other applications) was offered by Ford in some of the baddest muscle cars of the '60s as an option to try and rein in the big-block power of the day. Today, the Detroit Locker is still a viable option for a brutally strong differential that locks both wheels for optimum traction, but is still livable on the street due to its ability to automatically unlock for wheel speed differentiation. The Detroit Locker works by using two spring-loaded, driven clutch assemblies (in place of spider gears) that are mated to a centrally located spider assembly. The spider is driven by the main case and ring gear assembly. This means as long as the vehicle is going straight, the driven clutches are mechanically locked to the central spider, as if the axles are welded together. When differentiation is required, the outside wheel, which has a farther distance to travel, disengages the driven clutch on that side of the axle, while the inboard driven clutch stays mechanically locked to the central spider. The disengagement is what gives the Detroit Locker its famous click-click-click when turning at low speeds. The Detroit Locker is available for the 9-inch in 28-, 31-, and 35-spline options, as well as for the late-model 8.8-inch axle in both 28- and 31-spline.

Another electro-mechanical, magnetically controlled locker is the Eaton ELocker from Eaton Corporation. The ELocker differs from the ECTED Max we mentioned previously in that instead of being a limited-slip-based unit when "off," the ELocker instead acts like a traditional open differential when not engaged. The open-style diff is much easier on driveline parts and is also better on fuel economy, but when you need the traction abilities of a locker it is simply a push of a button away. When activated, the electromagnet creates a magnetic pull on a drag plate, which activates a ramping mechanism. This ramp acts as a one way clutch that engages the side gear, locking it to the differential housing. This all happens in a near instantaneous state for full torque equally transmitted to both wheels. Like the ARB locker, when the ELocker is engaged there is no differentiation for cornering. It's either locked or it is open, making the ELocker more suitable for drag racing and serious street use than for corner carving. The Eaton ELocker is currently available for 31-spline 8.8-inch axle configurations.

The Powertrax No-Slip locker is offered by Richmond Gear for the 8-inch 28-spline; 8.8-inch in 28- and 31-spline; and the 9-inch in 28-, 31-, and 35-spline configurations. The Powertrax unit is a unique locker in that it is installed into the vehicle's original open or limited slip differential housing in place of the OE guts. What this means is that you can easily upgrade your 8-inch open rear in your driveway with basic handtools, without affecting ring and pinion mesh, bearing preload, or other sensitive measurements. The Powertrax is designed to lock both rear axles together for full torque output to both axles, no matter the traction surface, yet allow for axle speed differentiation via disengagement of the side clutch, similar to the Detroit Locker explained earlier. With no friction clutches to wear out, no special setup tools required, and installation that can be accomplished at home in a little more than an hour, the Powertrax No-Slip locker is a great choice for street use. If you're looking for an all out dragstrip option (or serious street) Powertrax does offer its original Lock-Right, which is stronger, but also noisier.

Spools

If all you care about is 100 percent traction and going straight, then a spool is a viable option for your ride. While not considered street friendly due to the harsh strain a spool can put on the drive-line parts of your car, limited street use is possible. Currie Enterprises offers a 9-inch lightweight full spool in 28-, 31-, 33-, 35-, and 40-spline configurations, while a mini-spool is also available for the 8- and 9-inch rear as well in 28- and 31-spline configurations. The mini-spool gets installed into a traditional open differential housing (inset), so it makes it a great way to install a spool without messing with gear mesh, bearings, and so on.

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Yukon offers both a mini and full spool for several Ford applications. The full spool is made from 4130 chrome-moly steel and is heat treated for strength. A standard spool is offered, as are lightweight race models to reduce rotating mass. The full spool for 9-inch axles is available in 28-, 31-, 33-, 35-, and 40-spline models as well as the 8.8-inch rear with 31-spline axles (requires C-clip eliminators or 9-inch bearing end conversion). Mini-spools are offered for the 8.8-inch axle with 28- and 31-spline axles, the 8-inch with 28-spline axles, and 9-inch with both 28- and 31-spline axles. The mini-spool can be easily installed into an OE open differential without disturbing ring and pinion mesh.